It is known in the field of photography that silver halide crystal grains are useful for forming latent images by irradiation with visible light, ultraviolet light, or radiations such as .beta.-rays, neutron beams, and .gamma.-rays, and further forming visible images by developing the latent images. As such silver halide grains, various silver halide crystal grains such as silver iodide, silver bromide, silver chloride, silver iodobromide, silver iodochloride, silver chlorobromide, silver iodochlorobromide, etc., are used. Also, with respect to the form of these silver halide crystal grains, regular grains such as cubic form, octahedral form, tetrahedral form, dodecahedral form, etc.; irregular crystal grains such as spherical form, tabular form, indefinite form, etc.; and crystal grains of multicomposed structure having stratiform structure or epitaxial structure (junction-type structure) in the grains are known. That the halogen composition, the form, or the structure of the grains largely influences various properties of silver halide grains is not only clear from the descriptions on the properties of silver halide in Chapter 1 and Chapter 'of T. H. James, The Theory of the Photographic Process, 4th Edition, (Macmillan Publishing Co., Inc., New York), and the description of the form of silver halide in Chapter 3 of ibid., but also is well known based on many sources to persons skilled in the art.
The halogen composition of silver halide emulsions, the form of silver halide grains, and the grain sizes or grain size distributions of silver halide grains are properly selected according to the use of the photographic material for which the silver halide emulsion is used and the performance imparted to the photographic material. However, silver halide grains sufficiently satisfying the desired performance are not always obtained, and hence it has been of great interest for persons skilled in the art to obtain silver halide emulsions sufficiently satisfying the desired performance.
For example, regarding the photographic performance, high sensitivity, the occurrence of less fog, excellent graininess, desired gradation, etc., have been desired; regarding the processing performance, quickness and stability have been desired; and further silver halide emulsions having excellent stability with the passage of time and pressure resistance have been expected.
In particular, in the field of color photographic light-sensitive materials, quickness and stability in processing, as well as toughness of photographic materials in handling thereof have been strongly desired. Thus, it is very useful to provide silver halide emulsions having excellent properties in these points.
Silver halide emulsions have various features according to the kind of the halogen. For example, a silver chloride emulsion is low in sensitivity but is excellent in developing speed and suitable for quick processing. Also, the silver chloride emulsion is liable to form fog. On the other hand, a silver bromide emulsion is somewhat slow in developing speed, for forms less fog and also has a high sensitivity. A silver iodide emulsion is very difficult to develop, and is almost never used alone, but mixed crystals of silver iodide and silver bromide are particularly important for photographic materials having an excellent light-sensitivity.
Various techniques of utilizing these features of the various kinds of silver halides are known. For example, there are many reports about stratiform structures using core-shell type silver halide grains. Typically, in such a silver halide, the whole surface of the core is coated with one or more other silver halides. Japanese Patent Publication No. 18,939/81 describes that a core-shell type silver halide emulsion composed of silver bromide as the core and silver chloride as the shell has a high light-sensitivity of silver bromide and a quick developability of silver chloride, but in a mixed crystal type silver chlorobromide emulsion, both advantageous functions are inhibited. Also, West German Patent Application (OLS) No. 3,229,999 discloses that core-shell type silver halide grains formed by disposing a silver halide layer having at least 25 mole% silver chloride adjacent to a silver halide layer having a less content of silver chloride than the former are less in fog formation and good in pressure resistance.
Various techniques are also known about silver halide crystal grains having a different structure form the core-shell structure. For example, U.S. Pat. No. 4,094,684 discloses an emulsion containing silver halide grains formed by epitaxially growing silver chloride onto polyhedral silver iodide. Similarly, U.S. Pat. No. 4,463,087 discloses an emulsion containing a silver salt epitaxially grown onto host silver halide grains containing silver iodide surrounded by (111) crystal faces and a process for producing the same; and U.S. Pat. No. 4,471,050 discloses an emulsion containing silver halide host grains of a face-centered cubic crystal structure and non-isomorphic salts which are grown only at the edges or corners of the host grains. Furthermore, Japanese Patent Publication No. 24,772/83 describes cubic silver halide crystals having a different halide composition between the corner portions and the principal portion and also discloses that it is possible to selectively introduce impurities and to control the formation of crystal defects.
In this respect, it is described that when silver chloride is deposited onto octahedral silver bromide crystals, many small silver chloride crystals having (100) planes are formed on the eight (111) faces of the octahedron, and when the deposition of silver chloride is further continued, they are united to form faces as a cube, in C. Hasse, H. Frieser, and E. Klein, Die Grundlagen der Photographischen Prozesse mit Silberhalogeniden, Vol. 2, (Akademische Verlagsgesellschaft, Frankfurt an Main, 1968).
Also, it is reported by C. R. Berrry and D. C. Skillman, Journal of Applied Physics, Vol. 35, No. 7, p. 2165 (1964) that the deposition of silver chloride onto octahedral silver bromide causes an epitaxial growth of silver chlorobromide mixed crystals on the (111) faces thereof, and that the deposition of silver chloride on cubic silver bromide shows an epitaxial growth only at the corners or edges of the cube.
In these known techniques or knowledges described above, a silver halide such as silver chloride is epitaxially grown selectively at the corners or edges of crystals of other silver halide (such as silver bromide) or is grown on the (111) faces of the crystals; or, in the above-described core-shell type silver halide grains, a silver halide is uniformly grown over the whole surface of a core silver halide grain. However, epitaxial junction-type silver halide grains having a silver halide selectively epitaxially joined to and grown on the (100) faces of other silver halide crystals have not yet been known.